1. Field of the Invention
The present invention relates to an electron emission device, and in particular, to an electron emission device which has an improved electron emission structure to heighten the emission efficiency and lower the driving voltage.
2. Description of Related Art
Generally, electron emission devices are classified into a first type where a hot cathode is used as an electron emission source, and a second type where a cold cathode is used as the electron emission source.
Among the second type of electron emission devices known are a field emitter array (FEA) type, a surface conduction emission (SCE) type, a metal-insulator-metal (MIM) type, and a metal-insulator-semiconductor (MIS) type.
With the FEA type electron emission device, the electron emission regions are formed from a material that emits electrons under the application of an electric field, and that drives electrodes of cathode and gate electrodes are placed around the electron emission regions. When electric fields are formed around the electron emission regions due to the voltage difference between the two electrodes, electrons are emitted from the electron emission regions.
In some conventional FEA type electron emission devices, the electron emission regions are spindt-type with a sharp-pointed tip made commonly through depositing or sputtering molybdenum (Mo) in a vacuum. For example, U.S. Pat. No. 5,938,495 discloses a method of manufacturing field cold cathodes. The spindt-type electron emission region has a small size with a bottom diameter of about 0.5 μm, and a height of 0.5-1 μm.
A semiconductor fabrication process should be used to manufacture an electron emission device with the spindt-type electron emission regions. The processing steps are complicated with such highly specialized techniques, however, so that the production cost is increased, and it becomes difficult to enlarge the display area.
It has been recently proposed that the electron emission regions should be formed with a carbonaceous material having a low work function, such as carbon nanotube, graphite and diamond-like carbon, using a thick filming process like the screen printing. Electrons are easily emitted from the carbonaceous electron emission material on the surface of the electron emission regions so that the low voltage driving thereof can be made while allowing the display area to be enlarged.
However, with the electron emission device having the carbonaceous material-based electron emission regions, when an insulating material is screen-printed, dried and fired one or more times to form an insulating layer with a thickness of 5-30 μm, the height of the opening portion to be formed with the electron emission region, that is, the thickness of the insulating layer is established to be 5-30 μm. The thickness of the electron emission region formed through the screen printing, drying and firing within the opening portion, however, is established to be at best 3-4 μm.
Consequently, with the conventional electron emission device, the distance between the electron emission region and the gate electrode is enlarged so that the emission efficiency is deteriorated, and the driving voltage becomes heightened. Furthermore, as the opening portion has a relatively large height compared to the thickness of the electron emission region, some of the emitted electrons collide against the insulating layer so that the insulating layer is charged while distorting the trajectories of the electron beams. Furthermore, the emitted electrons partially collide against the gate electrodes, and are leaked so that the amount of electrons reaching the phosphor layers is decreased.
The electron emission regions are formed with a plane shape of a circle or a rectangle in accordance with the shape of the opening portions. The electric fields are not uniformly applied to the electron emission regions, but concentrated on the periphery thereof positioned closest to the gate electrodes. The electrons emitted from the periphery of the electron emission regions are diffused with a predetermined diffusion angle.
Consequently, the emitted electrons do not properly strike the target phosphor layers at the relevant pixels, but land on unintended incorrect color phosphor layers while light-emitting them, thereby deteriorating the screen image quality.